Generally, electric vehicles or hybrid electric vehicles (hereinafter, referred to as electric-driven vehicles) are driven in an electric-driven mode using an electric energy stored in a battery.
The battery used is generally a secondary battery capable of charging and discharging and due to repeated charging and discharging, leads to the degradation of the secondary battery and its performance deterioration. Therefore, when using a secondary battery, a technology for quantitatively evaluating the degree of battery capacity degradation is needed.
State of health (SOH) is a parameter quantitatively representing change of battery capacity characteristics caused by an aging effect and works for showing the degree of battery capacity degradation. Therefore, if SOH is known, a battery can be replaced at a proper time and can be protected from over-charging and over-discharging by controlling battery capacity for charge and discharge based on the battery usage time.
Meanwhile, since the change of battery capacity characteristics is reflected on the change of internal resistance of a battery, SOH of the battery can be estimated from the internal resistance and the battery temperature as well known in the art. In other words, the battery capacity at each internal resistance and each battery temperature is measured through charging and discharging experiments. Then, the measured capacities are expressed as relative numerical values based on an initial capacity of the battery to obtain a look-up table for SOH mapping. After that, internal resistance and battery temperature are measured under an actual use circumstance, and afterwards the SOH corresponding to the measured internal resistance and the temperature is mapped from the look-up table to estimate SOH of the battery.
In the above SOH estimating method, the most important thing is how accurately an internal resistance of a battery can be obtained. However, it is actually impossible to directly measure an internal resistance of a battery during a battery charging or discharging process. Therefore, generally, a battery voltage and a battery charging or discharging current are measured, and then a battery internal resistance is indirectly calculated according to Ohm's law. However, the indirectly calculated battery voltage is significantly different from an actual voltage of a battery due to IR drop effects. In addition, since a battery current are prone to measurement errors, the internal resistance simply calculated according to the Ohm's law and the SOH estimated from the internal resistance do not ensure sufficient reliability.
For reference, the IR drop effect means a phenomenon in which a battery voltage rapidly changes when a battery starts to discharge by connecting to a load or when a battery starts to charge by connecting to an external electric power. In other words, a battery voltage rapidly decreases when discharging is initiated, and a battery voltage rapidly increases when charging is initiated.
As another example of the SOH estimation method, there is a full discharge test, a chemical test and an Ohm test. In the full discharge test, SOH of a battery is estimated by fully discharging a fully charged battery cell and then measuring the capacity of the battery cell. However, this method interrupts the operation of a system connected to the battery and wastes the energy of the battery cell. In the chemical test, SOH of a battery is estimated by measuring corrosion of a substrate and electrolyte concentration of a lead-acid battery. In the Ohm test, SOH of a battery is measured by measuring resistance, conductance, and impedance. However, the aforementioned methods disadvantageously demand an invasive measurement of a battery. Therefore, a noninvasive method, which does not interrupt a connected system and not waste battery energy, is needed for electric-driven vehicles which actually use a secondary battery.
As another example of the SOH estimation method meeting the above requirements, it is also possible to estimate state of charge (SOC) of a battery by charging or discharging currents of the battery, and estimating SOH of the battery by using the estimated SOC. However, in this method, since measurement errors generated in the current measuring progress is continuously accumulated, the accuracy of the SOC used as a basis for estimating SOH of a battery is deteriorated as time goes. Therefore, it is difficult to estimate a reliable SOH of a battery.
As another example of the SOH estimation method, SOH of a battery may also be estimated by using a Kalman filter or an extended Kalman filter. However, since this method uses a complicated mathematical model to estimate SOH of a battery, not only is the calculation procedure too complicated, but a central processing unit with high specification is needed to implement the calculation.